Various parameters are employed to describe the accuracy of a coordinate measuring machine. The total accuracy of a coordinate measuring machine depends upon the repeatability, accuracy, and orthogonality of the machine axes. This accuracy, which is of most importance to the user, is referred to as volumetric accuracy.
Industry standards define methods to verify the volumetric accuracy of a coordinate measuring machine. One such standard is the ANSI B89 Standard. According to this standard, a test artifact is provided, constructed of two precision metal balls joined by a rigid member. When in use, this ball bar is held by an adjustable stand. The machine to be tested is programmed to measure a number of points on the surface of each of the two balls, and to calculate a center location for each ball. From this information, the apparent center-to-center ball bar length can be determined. This ball bar length is then compared to the known length of the ball bar in use. The ANSI B89 Standard calls for measuring the same ball bar in a number of different locations and angles in order to determine the entire volumetric accuracy of the coordinate measuring machine under test.
A known device for performing such a calibration operation is described in U.S. Pat. No. 4,435,905. The device comprises an elongate telescopic bar with a metal ball at each end. In use, each of the balls is retained in a magnetic socket provided on the probe holder and the workpiece table respectively. The probe holder is then driven in a circular path about the center of the ball retained in the socket on the workpiece table. A single axis transducer provided on the bar measures any variation in the center-to-center spacing of the balls, and thus determines the extent to which the probe holder path varies from its programmed circular path.
This known method and apparatus has significant drawbacks. On a typical servo-driven coordinate measuring machine, measurements are performed by positioning the coordinate measuring machine probe a short distance away from the surface under test. The probe is then activated to receive a touch signal. The coordinate measuring machine is then instructed to move the probe toward the surface under test. As soon as a contact signal is received from the probe, the Cartesian coordinates of the test surface are recorded, and motion is halted. Thus, coordinate measuring machines are not typically required to move to a predetermined space point to very high accuracy. Nor are they constructed for such tasks. In fact, the proportional/differential/integral motor control of many coordinate measuring machines have long time constants and may take several seconds to move within one or two microns of the desired location. Despite these shortcomings, existing ball bar calibration systems require the coordinate measuring machine probe holder to move precisely along a predetermined path.
In addition, existing ball bar systems often use a separate data output or computer to compare the recorded length to the actual length of the ball bar under test. These separate systems do not have access to the coordinate measuring machine motion controller or servo motor output values. While the coordinate measuring machine may not have located itself precisely at the requested location, current ball bar systems assume the location to have been reached perfectly, and attribute any discrepancy to coordinate measuring machine error.
The present invention is directed to overcoming one or more of the above-noted problems.
More specifically, an object of the present invention is to provide a coordinate measuring machine calibration method and apparatus which operates the coordinate measuring machine in the same manner it operates during a typical measurement cycle. In other words, the present invention provides a method and apparatus which do not require the coordinate measuring machine to reach a precise location, but only to record its precise location at the time of probe activation.
It is another object of the present invention to provide a ball bar calibration system which utilizes data from the coordinate measuring machine motion controller or servo motor output in determining volumetric accuracy.
It is yet another object of the present invention to provide a calibration method and apparatus capable of operating at substantially the same speed as the coordinate measuring machine is operated during a typical measurement cycle.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.